1. Field of the Invention
This invention relates to an improved process for the alkylation of olefins in an alkylation unit feed stream with a view to producing a higher yield of alkylate having a higher octane number, a lower acid consumption rate, and a lower energy consumption. This improved process comprises the steps of passing a feed stream comprised of olefins and paraffins through a membrane whereby the olefins and paraffins are separated into an olefin stream, comprised of olefins, and a paraffin stream, comprised of normal paraffins and isoparaffins. An isoparaffin sweep stream is used to facilitate transport of the olefins across the membrane and the resulting olefin/isoparaffin stream is then passed into an alkylation reactor. Within the reactor, the olefins react with the isoparaffins to form alkylate.
2. Description of the Prior Art
Alkylation is a process which is widely practiced in the petroleum refining industry to produce liquid gasoline from normally gaseous hydrocarbons. It involves the reaction of low molecular weight isoparaffins, such as isobutane with low molecular weight olefins such as ethylene, propylene, butylenes and pentenes (amylenes). A particularly advantageous process is one in which isobutane is reacted with butylenes. As conventionally practiced, the alkylation process is usually conducted at low temperatures in the presence of a strong mineral acid catalyst, e.g., about 40.degree. F. for sulfuric acid and about 90.degree. F. for hydrofluoric acid. The manner of conducting the process and the conditions at which optimum results are obtained are all well known to those skilled in the art. Ordinarily, the feed to the process is a mixture of light hydrocarbons such as that produced in a catalytic cracker or other refining operations and this feed usually contains varying amounts of n-paraffins, isoparaffins, and olefins. In addition, since the catalytic cracker feed to the unit does not ordinarily contain the amount of isoparaffins required to alkylate all the olefins in this stream, other streams from various refinery and outside sources, containing isobutane and normal butane, are also fed to the unit. However, such mixtures are not the most desirable feed to an alkylation unit in view of the n-paraffin content since it is only the isoparaffins which will catalytically alkylate. The n-paraffins do not react and serve merely as diluents in the reaction mixture.
Since best results are obtained when the isoparaffins are present in a substantial molal excess as compared to the olefins, economics dictate that the unreacted isoparaffins should be separated from the alkylate and be recycled back to the reactor. As mentioned earlier, an alkylation feed stream will ordinarily contain non-reactive n-paraffins as well as reactive isoparaffins. Consequently, the n-paraffins must be separated from the isoparaffins before the latter are recycled to the alkylation reactor since, otherwise, the n-paraffins would tend to accumulate and diminsh the efficiency of the reactor. In the n-butane-isobutane situation, separation is difficult because of the close proximity of the boiling points of the compounds and, up until now, it has been necessary to resort to expensive distillation equipment to accomplish this separation. Furthermore, a perfect separation between the normal and isoparaffins has been totally impractical with conventional distillation equipment and consequently a substantial quantity of the n-paraffins are circulated within the alkylation unit. The presence of the n-paraffins necessitates the treatment of larger streams by the deisobutanizer and the depropanizer which in turn results in a greater energy consumption. Difficult separations are also encountered when one tries to remove the n-paraffins from an ethane-ethylene feed, a propane-propylene feed, or a pentane-pentene feed. Accordingly, there is a need for a process that provides a method of separating the olefins from the paraffins, and facilitating the transport of the olefins by an isoparaffin sweep stream so that a higher yield of alkylate can be produced with a lower acid consumption rate, and a lower energy consumption.
Commonly assigned U.S. Pat. Nos. 3,758,603, 3,758,605 and 4,060,566 disclose membrane separation of olefins from paraffins where the membranes are capable of separating both liquids and gases. For best results, a permeable film membrane made of cellulose triacetate impregnated with silver nitrate is used. Generally, a sweep stream is used to facilitate transport of the olefins through the membrane.
Accordingly, it is the general object of this invention to provide an improved process for the alkylation of an olefin feed stream. More specifically, the object of this invention is to reduce the concentration of n-paraffins in the alkylation reactor so that a higher alkylate yield is possible with lower energy consumption.
Another object of this invention is to provide an improved process which will produce an alkylate having a higher octane number.
Still another object of this invention is to provide an improved process which has a lower acid consumption rate.
Still further, an object of this invention is to provide an improved process for the alkylation of olefins in an alkylation unit feed stream when the feed stream contains ethane-ethylene, propane-propylene, butane-butylene, pentane-pentene, or combinations thereof.
We have now found that the objects of this invention can be attained by passing a feed stream comprised of olefins and paraffins through a membrane whereby the olefins are separated from the paraffins into an olefin stream comprised of olefins, and a paraffin stream comprised of normal paraffins and isoparaffins. The olefins are facilitated in their transport across the membrane by a separate isoparaffin sweep stream and this combined olefin/isoparaffin stream is passed into an alkylation reactor wherein the olefins react with the isoparaffin to form alkylate. This process has the additional advantage of being physically incorporated into existing alkylation units with a minimal amount of engineering changes.